The present invention relates to an apparatus and method to measure magnetic properties of data recording media, particularly the coercivity of the recording media.
Rotating disc magnetometry (RDM) is a useful method for measuring the magnetic properties of magnetic recording discs. The method is both rapid and non-destructive. One challenge of constructing a useful, non-contact, rotating disc magnetometer is the availability of writing heads that are capable of fully saturating the relatively high coercivity materials used in high capacity magnetic storage systems. The coercivity of typical recording media used in disc drives is about 3000 Oe. It is projected that this number will rise within the next year to about 4000 Oe. Further increases to perhaps as much as 6000 Oe are anticipated over the next several years. Media with coercivities in excess of 4000 Oe are difficult to saturate with currently available large-scale writing heads operated at mechanical clearances of 0.075 mm from the disc surface. Therefore, if RDM is to be used in the near future to measure recording media with ever increasing coercivity vastly improved writing heads are required.
A rotating disc magnetometer generally comprises a rotating support to maintain the recording medium to be measured, a recording head containing a transducer that can be selectively positioned over the recording medium, and a programmable control unit. The transducer has many roles in measuring the magnetic properties of the recording medium. The transducer applies magnetic fields to write magnetic transitions (data) onto the recording media, senses the resulting fringing fields to read the data, and applies erasing magnetic fields to remove or erase data previously recorded.
Magnetic media performance depends largely upon magnetic properties of the recording layer. Important properties in this regard include the coercivity, the remanance-thickness product, and the coercive squareness or switching squareness. The coercivity is the magnetic intensity of an applied magnetic field sufficient to cause the recording media to undergo a transition from a state of magnetic saturation to a non-magnetized state. The remanance-thickness product indicates the density of magnetic poles in the remanent state and thus relates to the strength of the electrical signal recoverable from a magnetization transition. The coercive squareness or switching squareness describes the rate of change of magnetization as a function of an applied magnetic field.
There are several known approaches to testing magnetic media for their magnetic properties, including vibrating sample magnetometers for measuring magnetization as a function of an applied magnetic field, and hysteresis testers for measuring flux density as a function of an applied field. These devices, however, are destructive in that they require cutting or otherwise separating a finite element from the recording medium for measurement.
U.S. Pat. No. 4,847,558, assigned to the assignee of this application, discloses a non-destructive approach for determining coercive force (i.e., coercivity). In particular, a standard read/write head (magnetic induction transducer) supported on an air bearing, is used to generate currents of increasing strength and thereby apply erasing magnetic fields of increasing strength to previously recorded transitions. The magnetic intensities of the applied erasing fields are calculated based on an assumed linear current-field relationship as compared to the signal level before applying the erase current.
In order to write on 6000 Oe recording media, and therefore, to measure the magnetic properties of the recording medium, improvements in writer gap fields of 50% are required with the present technology. It is not likely that such a large improvement can be made. Therefore, an improved device and method of producing fully saturated magnetic transitions is required.
This invention is directed to an apparatus and of a method for measuring the magnetic properties of recording media. Particularly, the invention is directed to an apparatus and of a method for measuring the coercivity of disc recording media.
The measuring apparatus of the invention contains a directed thermal source to increase the temperature in a selected portion of recording media; an electromagnet to apply external magnetic fields of variable magnitude to the recording media; and a recording head to detect a magnetization signal resulting from magnetic transitions induced by the thermal source and external magnetic fields on the recording media. The apparatus may also include a programmer to coordinate the electromagnet, the thermal source and the recording head. The programmer may also be used to store and convert magnetization data, and calculate coercivity values for selected portions of the recording media.
The measuring apparatus will further contain at least one stage to movably adjust the position of the electromagnet and the thermal source to the selected portion of the recording media. Preferably, another stage is used to movably adjust the position of the recording head to the same selected portion of the recording media. A drive is used to position the recording medium such that the selected portion that is induced to undergo magnetic transitions by the external magnetic field and thermal source is subsequently rotated into a position so the magnetization signal can be detected by the recording head. In the preferred embodiment, the directed thermal source is a laser and the recording head is a replay transducer. The laser will have a characteristic wavelength between approximately 300 and 2000 nm, preferably between approximately 500 and 1000 nm. It is preferred that the electromagnet be a ring electromagnet such that the recording media passes through the poles of the ring electromagnet. The electromagnet should be of sufficient power to apply an external magnetic field between approximately 0.01 and 20 Tesla, preferably between 0.1 and 4 Tesla.
An embodiment of this invention is an apparatus comprising a directed thermal source to increase the temperature in a selected portion of a recording medium; an electromagnet to apply external magnetic fields to the selected portion of the recording medium; and means for detecting a magnetization signal resulting from magnetic transitions on the selected portion of the recording medium, wherein the magnetization signals are used to measure magnetic properties of the selected portion of the recording medium.
The measuring apparatus of the invention includes: a means for directing a thermal source, preferably a laser, and an external magnetic field to a selected portion of a recording medium; a means for detecting magnetization signals; a means for coordinating the thermal source, the external magnetic fields, and the detection means to the selected portion of the recording medium that is induced to undergo magnetic transitions; and a means for store magnetization data and calculating coercivity values for each selected portion of the recording medium.
The invention is also of a method to measure magnetic properties of a recording medium, the method including: applying a first (strong) external magnetic field such that an effective magnetic field at a selected portion of the recording media is greater than the coercivity of the selected portion of the recording media at ambient temperatures; applying a second (weak) external magnetic field of a magnitude less than the coercivity of the recording media at ambient temperatures; directing a thermal source, preferably a laser, to generate thermal gradients to lower the coercivity of the recording media and assist the inducement of magnetic transitions in the recording media; and detecting magnetization signals on the recording medium with a recording head. Preferably, a ring electromagnet capable of producing external magnetic fields between approximately 0.01 and 2 Tesla is used.
The method of the invention will further include: applying a series of DC erasing fields of increasing incremental magnitude to the recording media; adjusting the position of the electromagnet and the thermal source with respect to a selected portion of the recording media; adjusting the position of the recording head with respect to the selected portion; coordinating the thermal source, the electromagnet, and the recording head with a programmer; and rotating the recording medium with a drive mechanism such that the selected portion of the recording media that is induced to undergo magnetic transitions is subsequently detected by the recording head, preferably a replay transducer. Preferably, the programmer will also digitize and convert the magnetization signals from the recording head, store the magnetization data, and calculate the coercivity values for the selected portions of the recording media.